If you use the category_description
filter hook you can replace the title attribute for example:
function my_cat_title($category_description,$category){
return '';
}
add_filter('category_description','my_cat_title');
$cat_args = array('orderby' => 'name');
$cat_args['title_li'] = '';
$cat_args['exclude_tree'] = my_cat();
wp_list_categories(apply_filters('widget_categories_args', $cat_args));
remove_filter('category_description','my_cat_title');